Control moment gyroscopes (CMGs) are well known means of providing directional control of spacecraft. One example is described in U.S. Pat. No. 5,386,738 to Havenhill, the disclosure of which is incorporated herein by reference in its entirety. CMGs typically include a rotor, a motor to spin the rotor about a rotor axis, a gimbal, a gimbal torque motor to rotate the gimbal about a gimbal axis and a control system. The control moment gyroscope is mounted within the spacecraft along the axis in which torque will be induced. The rotor is mechanically supported in the gimbal and is rotated about the gimbal axis, which is normal to the rotor axis.
During operation of the CMG, the rotor is spun about its rotor axis by a motor at a predetermined rate. In order to induce a torque on the spacecraft, the gimbal torque motor rotates the gimbal and spinning rotor about the gimbal axis. The rotor is of sufficient mass and is spinning at such a rate that any movement of the rotor out of its plane of rotation will induce a significant torque around an output axis, which is normal to both the rotor axis and the gimbal axis. Torque around the output axis is transferred directly to the spacecraft through a support structure.
Typical CMGs provide torque in response to a gimbal-rate command, which neglects gimbal dynamics. Because of imperfections in sensing and actuation, commanding a gimbal rate does not result in the output torque desired but, rather, a close approximation thereof. Improving the match between desired and actual output torque requires high-precision tachometers, which represent considerable cost and are likely impractical for the desired very small CMGs.
The spacecraft has an attitude control system which may employ one or more CMGs. The bandwidth of the gimbal-rate control loop (typically about 20 Hz for using gimbal rate as feedback) represents an upper bound on the spacecraft agility by limiting the bandwidth of the attitude control system.